Tension motor control system



Dec. 14, 1965 E. H. DINGER TENSION MOTOR CONTROL SYSTEM Filed March l. 1960 mmFmZOFZmPOm om odi opos. .EME

INVENTOR. EDWARD H. DINGER,

ATTORNEY.

United States Patent O 3,223,906 TENSION MOTOR CONTROL SYSTEM Edward H. Dinger, Waynesboro, Va., assignor to General Electric Company, a corporation of New York Filed Mar. 1, 1960, Ser. No. 12,097 4 Claims. (Cl. S18-7) The invention relates to a tension control system and particularly to such a system that maintains the tension of a material at a predetermined magnitude as it is being reeled or wound up.

In the manufacture of certain materials, particularly those taking the form of long strips, a reeling or wind ing up of the material is usually required. Thus paper is usually manufactured in very long strips which are wound on a reel for handling. As the material is so wound, it should be kept under a predetermined tension to prevent its sagging and accumulating under too low a tension and to prevent its being stretched and torn under too high a tension. However, in order to maintain the desired tension, it is necessary to have some indication of the diameter of the material on the reel because, for a fixed amount of torque applied to the reel, the tension decreases as the diameter of the material on the reel increases. Previously, one way of getting an indication of the diameter has been to place a roller or similar element on the reel and to use the physical position of the element to indicate the reel diameter and hence the torque needed to maintain the desired tension. However, such an element had several disadvantages. One was the fact that the element physically contacted the material being wound and sometimes left a mark on the material. Another was the fact that the element created an obstruction which hampered the substitution of a new reel when one reel was full. And another was the fact that special circuitry or arrangements were required to convert the physical position of the element into a signal or indication that produced the necessary torque.

Accordingly, an object of the invention is to provide a tension control system which indicates reel diameter without physically contacting the reel.

Another object of the invention is to provide a tension control system that does not obstruct the reel.

Another object of the invention is to provide a tension control system that is adapted to be used with conventional control circuitry.

Another object of the invention is to maintain the tension of a material being wound on a reel at a predetermined magnitude without the necessity of physically contacting the material on the reel.

Briefly, these and other objects are accomplished in accordance with the invention by a system in which a signal indicative of the feed speed at which the material passes a feed device is compared with a signal indicative of the speed of a pulling or reeling device for the material. An error signal indicative of the relative values of the feed speed signal and of the pulling speed signal is produced by this comparison. This error signal is utilized to adjust the torque of the pulling device and at the same time to vary the pulling speed signal until the error signal reaches a value at which no further adjustment in the pulling torque is indicated or called for. The system is designed so that the pulling torque reaches the magnitude needed to provide the desired tension in ice the material at the same time that the error signal has a value indicative of no required adjustment in the pulling torque.

The invention may be better understood from the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the claims. In the drawing, the single ligure shows a schematic block diagram of a preferred embodiment of the tension control system.

System description In the figure, mechanical or physical connections and couplings are shown by dashed lines and electrical connections or couplings are shown by solid lines. The embodiment shown has been applied to an arrangement in which some material, such as newly manufactured paper, is being reeled or wound up by a windup reel 10 that may be provided with a core 12 on which the material is wound. The windup reel 10 rotates in the direction indicated by an arrow, and the material flows in the direction also indicated by an arrow. The material is arranged or placed so that it tiows past, or by, or through a feed device taking the form of one or more feed rolls 14. As shown in the figure, the material passes between the feed rolls 14 and these feed rolls 14 usually rotate in the direction indicated by two arrows. The feed rolls 14 are designed so that they may be retarded or driven so as to control the linear speed of the material. A feed speed tachometer 16 has its mechanical input element coupled to the feed rolls 14 so that it produces an electrical signal having a value or magnitude and polarity indicative of the speed and direction of rotation of the feed rolls 14. Since the feed rolls 14 are arranged to rotate with the iiow of the material, the output electrical signal of the feed speed tachometer 16 has a magnitude also indicative of the linear speed or velocity of the material as it passes between the feed rolls 14. The electrical output signal of the feed speed tachometer 16 is applied to respective input circuits of two separate comparison circuits 18, 20. These cornparison circuits 18, 20 may be any suitable known comparison circuit, each comparison circuit producing an error signal having a magnitude indicative of the relative values of two or more signals applied to input circuits of the comparison circuit. A feed speed potentiometer 22 is coupled to one of the input circuits of the comparison circuit 18 so as to provide a Variable voltage thereto. The feed speed potentiometer 22, as well as the other potentiometers to be mentioned, comprises a resistive or impedance element represented schematically by the curved line and a movable arm which engages the resistive or impedance element. The impedance element may be tapered to provide any desired voltage change for a given angular rotation of the movable arm. This feed speed potentiometer 22 is provided with a source of energizing potential, such as the direct current supply indicated. The nature of this energizing potential depends upon the nature of the comparison circuit 18 to which it is applied; that is, there are known comparison circuits which operate on either alternating or direct current signals. And while the embodiment shown in the figure contemplates the use of direct current signals,Vl

it is to be understood that the embodiment may operate equally well on other types of signals. An example of a comparison circuit which may be used in the system shown in the figure may be found in an article entitled Current-Controlled D.C. Motor Drive by E. H. Dinger which appeared at page 78 of Electrical Manufacturing, vol. 64, No. 2, August 1959. Any error signal produced by the comparison circuit 18 is applied to a feed roll drive device 24. This feed roll drive device 24 may take any one of a number of forms. Its funtion is to maintain the linear speed of the material at a constant Value called for by the feed speed potentiometer 22. It does so by operating on the error signal produced by the comparison circuit 18 and mechanically transforming this error signal into a mechanical function which serves to change the driving or restraining elfect applied to the material flowing past or through the feed rolls 14. Thus, the feed roll drive device 24 could take rthe form of a drive, such as a motor, or a brake, such as a generator, to control the feed rolls 14 and thereby permit the material to flow more or less freely, and thereby maintain the linear speed of the material at a constant value.

The windup reel 10 is mechanically coupled to a reel speed rtachometer 26 which is similar in function and structure to the feed speed tachometer 16. The reel speed tachometer 26 produces an electrical signal having a magnitude or value indicative of the angular speed of rotationof the windup reel 10. This output signal of the reel speed tachometer 26 is applied to the comparison circuit 20 through a variable feedback potentiometer 28. The feedback potentiometer 28 serves an important function in the operation of the invention as will be explained. The comparison circuit 20 receives the signals from the feedback potentiometer 28 (these signals being proportional to the reel speed tachometer 26 signals) and the signals from the feed speed tachometer 16, and produces an error signal indicative of the relative values of these two signals. This error signal is applied to a drive means, such as the potentiometer motor 30. The function of the potentiometer motor 30 is to produce mechanical motion (rotary in the embodiment shown) in a direction and over a range indicative of the polarity and the magnitude of the error signal produced by the comparison circuit 20. The mechanical motion of the ptentiometer motor 30 is coupled to the movable arm or element of the feedback potentiometer 28, and is also coupled to the movable arm or element of a iield potentiometer 32. The mechanical motion of the potentiometer motor 30 is also coupled to the movable arm or element of an acceleration potentiometer 34. The impedance element of the field potentiometer 32 is coupled in the field circuit of a reel motor which drives or turns the windup reel 10. As shown in the figure, this reel motor comprises a reel motor armature 36 and a reel motor field 38. The reel motor may be a conventional direct current motor in which its armature 36 and field 38 may have variable voltages applied thereto. The rcel motor armature 36 has its shaft mechanically coupled to the windup reel as indicated. Thus, as the reel motorl armature 36 rotates, the windup reel 10 and the reel speed tachometer 26 also rotate. The reel motor field circuit comprises a source of direct current potential with which the impedance element of the ield potentiometer 32 and the reel motor field 38 are serially connected. The lield potentiometer 32 has a direct connection between its movable arm and one side of its impedance element so that lmore or less of the impedance element is shunted out or bypassed by the movable arm as the potentiometer motor 30 rotates. The reel motor armature 36 is provided with a suitable variable energizing potential from an armature control circuit 40. The armature control circuit 40 may take any suitable form, such as the control circuit shown in the same article in LElectrical Manufacturing referred to previously.

The armature control circuit 40 includes an armature current feedback circuit which provides a servo or closed loop control system so that proper operation of the reel motor armature 36 is provided. A comparison circuit 42 is provided which is similar to the other comparison circuits 18, 20 and which is designed to receive three input signals. The comparison circuit 42 produces an error signal indicative of the difference between the armature current feedback signal on the one hand and between the algebraic sum of the two other input signals on the other hand. The error signal is applied to the armature control circuit 40 which provides a current for the reel motor armature 36 that has a magnitude and polarity indicative of the error signal. The second signal applied to the comparison circuit 42 comes from a tension potentiometer 44, this tension potentiometer 44 being provided with a suitable energizing signal such as the source of direct current supply indicated. The third signal applied to the comparison circuit 42 comes from an acceleration potentiometer 34 when the contacts of an acceleration relay 46 are closed. The acceleration relay 46 functions in a manner which will be explained.

System operation With the system just described in operation, the reel motor rotates the windup reel 10 so that the material iiows past the feed rolls 14 in the direction indicated and is reeled or wound on the windup reel 10. With a given amount of current flowing through the reel motor armature 36 as determined by the setting of the tension potentiometer 44, and with a given amount of current flowing through the reel motor eld 38 as determined by the setting of the eld potentiometer 32, a iixed or substan. tially fixed torque is applied to the windup reel 10. As the material continues to be reeled or Wound on the windup reel 10, the diameter or the effective diameter and hence the circumference) of the material on the windup reel 10 increases. If the linear speed of the material is held fixed by the feed roll drive device 24 and its associated elements, the angular speed of the windup reel 10 must decrease since it winds more material with each succeeding revolution. Thus, the angular speed of the windup reel 10 (and hence the reel speed tachom eter 26 signal) vary as a function of the effective diameter of the windup reel 10. The relationship of the force on the material and the torque and radius of the windup reel 10 is as follows:

Force radius=torque; or force=torque/radius Thus, as the effective radius in the above relation increases, and if the torque is fixed, a diminishing force is applied at the point of effective pull on the material (this point being at the radial distance of the material from the center of the windup reel 10). It will be seen that since less force is applied to the material as the effective reel diameter increases, the tension on the material .is likewise decreased. However, it is quite desirable in a number of applications that this tension be maintained substantially constant. An example of such an application is the reeling up of newly manufactured paper.

As mentioned above, as the reel motor continues to turn with a fixed amount of torque (this torque being fixed by the given amounts of current flowing in the reel motor armature 36 and field 38), and wind up more material, the reel motor will turn more slowly because the diameter of the'material on the reel 10 is increasing. As the reel motor turns more slowly, the magnitude or value of the signal provided by the feedback potentiometer 28 (as derived from the reel speed tachometer 26) decreases. In accordance with the invention, this signal is compared in the comparison circuit 20 with the usually fixed signal from the feed speed tachometer 16 to produce an error signal. This error signal has a polarity and magnitude such that it causes the potentiometer motor 30 to rotate a given amount in one direction, say counter clockwise. The counterclockwise rotation of the potentiometer motor 30 causes the movable arm or element of the feedback potentiometer 28 and the movable arm or element of the field potentiometer 32 to also rotate in a counterclockwise direction. As the movable element of the feedback potentiometer 28 so rotates, it serves to provide an increased signal magnitude from the reel speed tachometer 26 to the comparison circuit 20. This increasing signal magnitude, when compared in the comparison circuit 20, produces an error signal which calls for less and less movement of the potentiometer motor 30. And as the movable element of the field potentiometer 32 rotates in the counterclockwise direction, the movable element tends to shunt or bypass more of the impedance element of the field potentiometer 32 with the result that an increased voltage is applied to the reel motor field 38. This reel motor field 38, with the increased voltage applied to it, provides the reel motor with additional torque, thus providing the desired force, and hence tension, on the material being wound or reeled on the windup reel 10. The resistive element of the field potentiometer 32 may be designed or tapered to provide a change in the voltage on the reel motor field 38 such that the torque varies in the desired manner with respect to rotation of the movable element.

It should be pointed out that the additional torque of the reel motor provides additional force, all of which is absorbed in maintaining the tension in the material at the desired magnitude. The additional torque does not cause the windup reel to rotate at a higher angular velocity or speeld because the feed roll drive device 24 with its associated elements maintains the feed speed of the material at a constant value. It should also be pointed out that if an increased signal magnitude were not provided by the feedback potentiometer 28 to the comparison circuit 20, the comparision circuit would be constantly comparing unequal signals (i.e., the fixed speed tachometer 16 signal and the otherwise decreasing reel speed tachometer 26 signal) to provide an error signal which would continuously cause the potentiometer motor 30 to rotate. Thus, the movable element of the field potentiometer 32 would be constantly rotated and would continually vary the torque applied to the reel motor and the windup reel 10. In accordance with the invention and with a proper design however, the movable element of the feedback potentiometer 28 may be rotated to a position so as to provide the comparison circuit 20 with a signal which, when compared with the signal from the feed speed tachometer 16, produces an error signal (usually zero) which calls for no further movement of the potentiometer motor. Thus, the feedback poten tiometer 28 serves the important function of preventini further increases in torque produced by the reel motor for a single change in the angular speed of the windup reel 10. As the material continues to be wound on the windup reel 10, the effective diameter of the material continually increases so that the potentiometer motor 30 rotates continually or in steps to keep up with the increased diameter of the material on the windup reel 10. This, the tension control system in accordance with the invention indirectly, and without physically contacting the material on the windup reel 10, provides a measure or indication of the effective diameter of the material on the windup reel 10. And, this indirect measurement of the diameter is used to maintain, as explained above, the tension or force on the material at a predetermined fixed or substantially fixed magnitude.

While the operation as described thus far comprises an important feature of the invention, the additional refinements shown in the figure will also be discussed. If it is desired to change the force or tension on the material flowing past or through the holdback rolls 14 to the windup reel 10, the torque on the reel motor may be changed by moving the movable element of the tension potentiometer 44 in a direction which provides for an appropriate change in the armature current. If an increase in tension were desired, the movable element would be rotated to provide an increased signal for application to the comparison circuit 42. With such a rotation, an error signal would be produced and applied to the armature control circuit 40 so as to increase the current through the reel motor armature 36. This would increase the torque on the reel motor, and this increased torque would increase the tension on the material flowing to the windup reel 10. The linear speed of the material flowing between the feed rolls 14 is maintained at its set value because of the comparison circuit 18 and the feed roll drive device 24. As the torque on the windup reel 10 is increased, the material may, for a brief instant, tend to move at a higher linear speed. This higher linear speed is accompanied by an increased output signal from the feed speed tachometer 16 which, when compared with the fixed signal provided by the feed speed potentiometer 22, produces an error signal that causes the feed roll drive device 24 to restrain the material more by an amount needed to maintain the linear speed constant. If a decrease in tension were desired, the movable element of the tension potentiometer 44 would be rotated to provide a decreased signal for application to the comparison circuit 42. In this case, an error signal would be produced and applied to the armature control circuit 40 so as to decrease the current through the reel motor armature 36, thus decreasing the torque on the reel motor. This decreased torque would decrease the tension on the material flowing to the windup reel 10. In this case, the material may, for a brief instant, tend to move at a lower linear speed. This lower linear speed is accompanied by a decreased output signal from the feed speed tachometer 16 which, when compared with the signal provided by the feed speed potentiometer 22, produces an error signal that causes the feed roll drive device 24 to restrain the material less (or even drive the material) by an amount needed to maintain the linear speed constant. Thus, the feed speed tachometer 16 and its associated elements maintain the set linear speed even when changes in tension are called for. Since the linear speed of the material is prevented from changing, the reel speed tachometer 26 and its associated circuitry remain unaffected until the effective reel diameter changes to produce a change in the angular speed or velocity of the windup reel 10. With such a change, the reel speed tachometer 26 and its associated circuitry then function in the manner explained above to maintain the desired tension.

If a change in the linear speed of the material is desired, the movable element of the feed speed potentiometer 22 may be rotated to provide the appropriate change. As an example, assume that the movable element of the feed speed potentiometer 22 is moved in a direction calling for an increase in the linear speed of the material. The comparison circuit 18 produces an error signal which causes the feed roll drive device 24 to operate so that the feed rolls 14 restrain the material less. Thus, for a previous amount of torque applied to the windup reel 10, the material is permitted to move at an increased linear speed. This increased linear speed of the material causes the feed speed tachometer 16 to produce an increased signal until such time as the comparison circuit produces an error signal calling for no further increase in speed. Since the feed rolls 14 restrain the material less, there is a decreased resistance or drag on the windup reel 10. This decreased resistance or drag permits the windup reel 10 to accelerate, and it does so until such time as the windup reel 10 rotates at the angular speed necessary to maintain the previous predetermined tension on the material. During this .acceleration or increase in the linear speed of the material, there.

may be a momentary decrease in the tension on the material until the windup reel 10 has accelerated t-o the new speed. However, if the increase in the linear speed of the material is made gradual, the change in tension in the material may ybe held to a very low magnitude. If the linear speed is to be reduced, the fee-d speed potentiometer 22 is adjusted accordingly and effectively causes the feed roll drive device 24 to operate so that the feed rolls 14 restrain the material more. This restraining of the material serves to decrease its linear Spee-d until such time as the Icomparison circuit 18 produces an error signal calling for no further decrease in speed. The increased restraint on the material causes the windup reel to decelerate, and it does so until such time as the windup reel 10 rotates at the angular speed necessary to main tain the previous predetermined tension on the material. During this deceleration or decrease in the linear speed of the material, there may be a momentary increase in the tension of the material -until the windup reel 10 has decelerated to the new speed. Again, if the decrease in the linear speed of the material is made gradual, the change in tension in the material may be held to a very low magnitude. But regardless of the change in linear speed of the material, the potentiometer motor 30 is not affected because both the Afeed speed tachometer 16 and the reel speed tac-hometer 26 lare varied t-o substantially the same degree so that the comparison circuit 20 produces the same error signal (usually zero) calling for no change in the operation or position of the potentiometer motor 30.

In the applications where a quick increase in the linear speed of the material may be called for, the acceleration potentiometer 34 and the acceleration relay 46 may be provided to supply additional torque to the windup reel 10 which may be needed to accelerate the angular velocity of the windup reel 1t) to provide the increased feed speed of the material, and also to maintain the tension inthe material at the same predetermined value. Circuit means not shown may be connected to the feed speed potentiometer 22 so that when the movable element of this potentiometer 22 is moved in a direction calling for additional feed speed, the winding of the acceleration relay 46 is energized to cause the contacts associated with this winding to close and permit the acceleration potentiometer 34 to supply an additional signal to the comparison circuit 42. The movable element of the acceleration potentiometer 34 is positioned at a point depending upon the rotation of the potentiometer motor 30. By a suitable design of the acceleration potentiometer 34 and by a suitable selection of a direct current voltage the acceleration potentiometer 34 cany be made to continually present an acceleration signal which takes the speed and di* ameter of the windup reel lil into consideration. This additional signal may have a magnitude or a value such as to increase the armature current to provide the necessary acceleration of the reel motor.

As will be appreciated by persons skilled in the art, the controlV system in accordance with the invention may be used in a number of different applications and in addition, may be used in many forms. For example, certain refinements, such as the acceleration potentiometer and relay circuit, may be eliminated or changed without departing from the scope of the invention. Thus, the invention may be modified to be applied to systems in which material is wound from instead of on a reel. In such a system, the reel from which the material is taken could either be driven or retarded in accordance with the angular speed of the reel (and hence the effective reel diameter) to maintain the tension of the material at the desired magnitude. However the system of the invention is used, it may provide a fixed tension for material being wound or unwound from a reel without physically contacting the material to ascertain its diameter. Furthermore, the system may be used with conventional control circuitry. And finally, it is to be understood that modifications may be made by persons skilled in the art without departing from the spirit of the invention or from the Scope of the claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

ll. A system for maintaining the tension of a material at a predetermined magnitude as it is pulled by a reeling device past a feed drive device, comprising first means for producing a first signal indicative of the linear speed of said material past said feed drive device, second means for producing a second signal indicative of the angular speed of said reeling device, a variable potentiometer coupled to said second means for varying the magnitude of said second signal, comparing means coupled to said first means and to said potentiometer for comparing said first signal with said second signal as, varied by said potentiometer and producing an error signal indicative of the relative values of said first signal and said varied second signal, first drive means coupled to said reeling device for operating said reeling device, second drive means coupled to said comparing means and operated in response to said error signal, means coupling said second drive means to said potentiometer and to said first drive means for operating said potentiometer and said first drive means in response to changes in said error signal, first independent means coupled to said first drive means for independently varying the torque of said first drive means, and second independent means coupled to said feed drive device for independently varying the braking and acceleration applied by said feed drive device to said material.

2. The invention as defined in claim 1, having in addition a second variable potentiometer operatively coupled to said second drive means and selectively coupled to said first independent means for applying an acceleration signal thereto.

3. A system for maintaining the tension of a material at a predetermined magnitude as it is pulled by a reeling device past a feed device, comprising first means for producing a first signal indicative of the speed of said material past said feed device, second means for producing a second signal indicative of the speed of said reeling device, a variable potentiometer coupled to said second means for varying the magnitude of said second signal, comparing means coupled to said first means and to said potentiometer for comparing said first signal with said second signal as varied by said potentiometer and producing an error signal indicative of the relative values of said first signal and said varied second signal, first drive means coupled to said reeling device for operating said reeling device, second drive means coupled to said comparing means and operated in response to said error signal, means coupling said second drive means to said potentiometer and to said first drive means for operating said potentiometer and said first drive means in response to changes in said error signal, and independent means coupled to said first drive means for independently varying the torque of said first drive means.

4. A system for maintaining the tension of a material at a predetermined magnitude as it is pulled by a reeling dev1ce past a feed device, comprising first means for producing a first signal indicative of the speed of said material past said feed device, second means for producing a second signal indicative of the speed of said reeling dev1ce, a variable potentiometer coupled to said second means for varying the magnitude of said second signal, comparing means coupled to said first means and to said potentiometer for comparing said first signal with said second signal as varied by said potentiometer and produclng an error signal indicative of the relative values of said first signal and said varied second signal, first drive means coupled to said reeling device for operating said reeling device, second drive means coupled to said comparing means and operated in response to said error signal, means coupling said second drive means to said potentiometer and to said first drive means for operating said potentiometer and saidv first drive means in responsel to changes in said error signal, and independent means coupled to said feed device for independently varying the OTHER REFERENCES breaking andlacceleration applied by sald feed device to German application, 1,067,920, October 29J 1959, Sal matena Class 318, Sub. 6 (2 pp. spec., l sht. dwg).

References Cited by the Examiner 5 German 2199116311011, 1,107,790, May 31, 1961, ClaSS 318, Sub. 6 (2 pp. spec., 1 sht. dwg.).

UNITED STATES PATENTS Germany, E 9,220 VIII b/ 21e, Mar. 29, 1956. 2,325,381 7/1943 Edwards et al. 318-7 X 2,366,148 12/1944 Montgomery 318-327 X MERVIN STEIN, Primary Examiner 2,590,666 3/1952 Winchester et a1. 318-7 2,917,689 12/1959 Abell M 318 6 10 MILTON O. HIRSHFIELD, ORIS L. RADER,

2,943,309 7/1960 Garrett 31:3 6 RUSSELL C- MADER, Examiners- 

1. A SYSTEM FOR MAINTAINING THE TENSION OF A MATERIAL AT A PREDETERMINED MAGNITUDE AS IT IS PULLED BY A REELING DEVICE PAST A FEED DRIVE DEVICE, COMPRISING FIRST MEANS FOR PRODUCING A FIRST SIGNAL INDICATIVE OF THE LINEAR SPEED OF SAID MATERIAL PAST SAID FEED DRIE DEVICE, SECOND MEANS FOR PRODUCING A SECOND SIGNAL INDICATIVE OF THE ANGULAR SPEED OF SAID REELING DEVICE, A VARIABLE POTENTIOMETER COUPLED TO SAID SECOND MEANS FOR VARYING THE MAGNITUDE OF SAID SECOND SIGNAL, COMPARING MEANS COUPLED TO SAID FIRST MEANS AND TO SAID POTENTIOMETER FOR COMPARING SAID FIRST SIGNAL WITH SAID SECOND SIGNAL AS VARIED BY SAID POTENTIOMETER AND PRODUCING AN ERROR SIGNAL INDICATIVE OF THE RELATIVE VALUES OF SAID FIRST SIGNAL AND SAID VARIED SECOND SIGNAL, FIRST DRIVE MEANS COUPLED TO SAID REELING DEVICE FOR OPERATING SAID REELING DEVICE, SECOND DRIVE MEANS COUPLED TO SAID COMPARING MEANS AND OPERATED IN RESPONSE TO SAID ERROR SIGNAL, MEANS COUPLING SAID SECOND DRIVE MEANS TO SAID POTENTIOMETER AND TO SAID FIRST DRIVE MEANS FOR OPERATING SAID POTENTIOMETER AND SAID FIRST DRIVE MEANS IN RESPONSE TO CHANGES IN SAID ERROR SIGNAL, FIRST INDEPENDENT MEANS COUPLED TO SAID FIRST DRIVE MEANS FOR INDEPENDENTLY VARYING THE TORQUE OF SAID FIRST DRIVE MEANS, AND SECOND INDEPENDENTLY VARYING THE BRAKING FEED DRIVE DEVICE FOR INDEPENDENTLY VARYING THE BRAKING AND ACCELERATION APPLIED BY SAID FEED DRIVE DEVICE TO SAID MATERIAL. 